Conventionally, for example, as a technology for generating methane gas by causing, with an anaerobic microorganism, methane fermentation on organic substances containing a large amount of solid such as food residues, livestock manure and woody raw materials, a system is known that has a configuration in which the organic substances are pulverized or crushed to be sieved into predetermined particle diameters and in which thereafter methane fermentation is caused (for example, see Japanese Laid-open Patent Publication No. 2010-142735).